The nematode resistance gene (Mi) of tomato is known to be closely linkded to the gene encoding acid phosphatase-1 (Apase-1.sup.1) on chromosome 6 (Rick, C.M., in Genetic Maps (1984) 3:474, Cold Spring Harbor Press). Apase-1.sup.1 is a variant found in domesticated tomatoes presumably introduced when the gene encoding resistance to the nematode Meliodogyne incognita was transferred from the wild species L. peruvianum (Rick, C.M., et al, Tomato Genetics Cooperative Reports (1974) 24:25); and, indeed, Apase-1.sup.1 is used as a selection marker in breeding nematode resistance (Medina-Filho, H.P., Ph.D. Thesis (1980) University of California, Davis).
Resistance to root knot nematodes is extremely important in a number of plants, including broadleaf crops such as tobacco, cotton, tomato, cucurbits, and other vegetables, fodder crops, and ornamentals. Nematode infestation results in a shallow and knotted root system leading to reduced feeder root development and reduced growth. Current methods of control, including treatment of soils with nematocides and crop management, are expensive and have undesirable environmental consequences in some instances. However, control of nematodes by providing resistance to the plant directly through the Mi gene is environmentally safe and, over the long run, inexpensive.
While the Mi gene is defined as a single locus and mapped on chromosome 6 of the tomato genome, the product of this gene is currently unknown. However, it is understood that transfer of this gene confers resistance to root-knotting nematodes, and that it resides close to a region encoding an acid phosphatase.
Acid phosphatases in general are present in a variety of forms and are characterized by their ability to hydrolyze phosphate monoesters at low pH. A number of these enzymes have been studied. Previous work with Apase-1.sup.1 shows that it is inherited as a single locus, Aps1, close to the Mi locus (Forbes J. F. , et al, Tomato Genetics Cooperative Reports (1976) 26:7-8).